1. Field of the Invention
The present invention relates to a mechanism for preventing slack in a printer carbon ribbon, and more particularly to a mechanism for preventing slack in the carbon ribbon of a thermal transfer printer when the printable medium is fed in the reverse direction from that in ordinary printing.
2. Description of the Prior Art
One type of conventional printer using the thermal transfer printing system employs a continuous label strip consisting of a large number of labels provisionally attached to a backing strip via an adhesive layer or a continuous tag strip without an adhesive layer and prints characters on the labels or tags by thermal transfer using a thermal transfer carbon ribbon.
Since the carbon ribbon is extremely thin, it tends to snake or develop slack as it is being fed. This has an adverse effect on printing performance.
Mechanisms for preventing carbon ribbon snaking and slacking are taught by Japanese Patent Public Disclosure Nos. Sho 60(1985)-157,440, Sho 60(1985)-193,683, Sho 60(1985)-204,560 and the like.
Typical of the conventional mechanisms for preventing slack in a printer carbon ribbon is that of Japanese Patent Public Disclosure No. Sho 60(1985-193,683) schematically illustrated in FIG. 3 herein. This is a mechanism by which the rotation of a carbon ribbon take-up shaft during carbon ribbon take-up is applied to a carbon ribbon feed shaft 14 to bias it in the reverse direction from that for feeding the carbon ribbon.
The ordinary conventional printer i shown in the overall schematic view of FIG. 3 comprises a printer frame 2, a feed shaft 6 for a label strip 5 consisting of a backing strip 3 having a large number of labels 4 (the printable media) provisionally attached thereon, a printing mechanism 9 having a thermal printing head 7 and a platen 8, a peel-off mechanism 11 having a peel-off plate 10, a backing sheet take-up shaft 12, a ribbon feed shaft 14 for a thermal transfer carbon ribbon 13, a carbon ribbon take-up shaft 15, a drive motor 16, a label strip sensor 17, and a carbon ribbon sensor 18.
The drive motor 16 provides the power for feeding the label strip 5 from the feed shaft 6 and the thermal transfer carbon ribbon 13 from the carbon ribbon feed shaft 14. The labels 4 on the backing strip 3 are printed with prescribed characters by supplying the thermal printing head 7 with printing signals.
The printed label strip 5 passes to the peel-off mechanism 11 where only its backing strip 3 is turned sharply at the peel-off plate 10 so as to peel the labels 4 off the backing strip 3.
The backing strip 3 removed of the labels 4 is taken up by the backing sheet take-up shaft 12 and the thermal transfer carbon ribbon 13 is taken up by the carbon ribbon take-up shaft 15.
A reverse rotation mechanism 19 is provided between the carbon ribbon feed shaft 14 and the carbon ribbon take-up shaft 15 for applying the rotation of the carbon ribbon take-up shaft 15 during take-up of the carbon ribbon 13 to the carbon ribbon feed shaft 14 to bias it in the reverse direction from that for feeding the carbon ribbon.
More specifically, a coil spring belt 22 (a coil spring formed into an endless belt) is fitted on a pulley 20 of the carbon ribbon feed shaft 14 and a pulley 21 of the carbon ribbon take-up shaft 15, whereby the rotation of the carbon ribbon take-up shaft 15 when it is driven to take up the carbon ribbon 13 is applied to the carbon ribbon feed shaft 14 in the reverse direction from that for feeding out the carbon ribbon 13.
So as to ensure that the carbon ribbon feed shaft 14 will be able to rotate in the direction for feeding the carbon ribbon 13 notwithstanding the application of the reverse rotational force thereto, a slip mechanism 23 is provided between the carbon ribbon feed shaft 14 and the pulley.
Since the provision of the reverse rotation mechanism 19 allows the carbon ribbon 13 to be fed toward the printing mechanism 9 while applying a prescribed tensile force to the carbon ribbon 13 by pulling it in the direction opposite to the feed direction, slacking of the carbon ribbon 13 is prevented.
For utilizing the printing area of the labels 4 to the utmost, however, thermal transfer printers for printing labels etc. are sometimes designed so that, as shown in FIG. 4, immediately upon completion of the printing of a given label 4, the label strip 5 is retracted by a distance D in the direction opposite to its forward feed direction for enabling the printing position of the printing mechanism 9 to be located as close as possible to the leading edge of the next label 4.
This leads to a problem because reversing the direction of rotation of the drive motor 16 for achieving the retraction not only retracts the label strip 5 but also retracts the carbon ribbon 13 in the direction of the carbon ribbon feed shaft 14, with the result that slack is produced in the carbon ribbon 13.
Moreover, the reverse rotation mechanism 19, which normally operates to draw the carbon ribbon 13 back in the direction opposite to its feed direction, i.e. in the direction of the retraction, has the opposite effect during retraction of the label strip 5 and, therefore, the carbon ribbon 13 develops even more slack.
In addition, although the sensor 18 for the carbon ribbon 13 can be of an optical type which senses reflected or transmitted light in the case where a black carbon ribbon 13 is used for black printing, it has to be a mechanical sensor such as a limit switch if, as has recently become common, a multi-color ribbon is used, since such a ribbon is difficult to detect optically. However, since such a mechanical sensor cannot properly detect a slack carbon ribbon, the printer is apt to malfunction.